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A polarizing filter, used both in color and black and white photography, can be used to darken overly light skies. Because the clouds are relatively unchanged, the contrast between the clouds and the sky is increased. Atmospheric haze and reflected sunlight are also reduced, and in color photographs overall color saturation is increased. Polarizers are often used to deal with situations involving reflections, such as those involving water or glass, including pictures taken through glass windows (this uses the phenomenon of Brewster's angle).
The benefits of polarizing filters are largely unaffected by the move to digital photography: though software post-processing can simulate many other types of filter, most of the optical effects of controlling polarization at the time of capture (particularly where reflections are involved) simply cannot be replicated in software.
http://upload.wikimedia.org/wikipedi...rPolarizer.jpg
The effects of a polarizer on the sky in a color photograph. The picture on the right has the filter.



http://upload.wikimedia.org/wikipedi...-polariser.jpg
In the first picture, the polarizer is rotated to minimize the effect, and in the second it is rotated 90° to maximize the effect - almost all reflected sunlight is eliminated.



http://upload.wikimedia.org/wikipedi...ough_Glass.jpg
On the bottom you can see how the window is reflecting the outside environment, while on the top the filter is turned 90° making it possible to see through the window.



http://upload.wikimedia.org/wikipedi...Vegetation.jpg
Polarizers are often used to improve the appearance of vegetation. In this picture the polarizing filter is used on the left.



There are two types of polarizing filters generally available, linear polarizers and circular polarizers (or CPL filters). With the exception of how they interact with some autofocus and metering mechanisms, they have exactly the same effect. Both transmit one of two states of linearly polarized light. The difference is that a circular polarizer alters the light leaving the filter and entering the camera by using a quarter-wave plate to circularly polarize that light. This has the same effect photographically as a linear polarizer, reducing glare in the scene. The metering and auto-focus sensors in certain cameras, including virtually all auto-focus SLRs, will not work properly with linear polarizers because the beam-splitters used to split off the light for focusing and metering are polarization-dependent. Circular polarizers work with all types of cameras, because mirrors and beam-splitters reflect both circular polarizations equally.[citation needed]

[edit] Neutral density

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